Automated power generation, transmission and distribution systems rely on various types of communication, e.g., control data exchanged between substations and a remote control center, protection data shared within the bay level, sample data sent from CTs/VTs (current transformers/voltage transformers) to intelligent electronic devices, control commands sent from intelligent electronic devices to switch gear, etc. An intelligent electronic device is a microprocessor-based controller of power system equipment, such as circuit breakers, transformers, capacitor banks, etc. Intelligent electronic devices receive data from sensors and power equipment, and can issue control commands, such as tripping circuit breakers if they sense voltage, current, or frequency anomalies, or raise/lower voltage levels in order to maintain the desired level. Common types of intelligent electronic devices include protective relaying devices, on load tap changer controllers, circuit breaker controllers, capacitor bank switches, recloser controllers, voltage regulators, etc. Some local communication, e.g., sensors reporting voltage and current sample values to an intelligent electronic device(s) in the same substation, goose messages exchanged between intelligent electronic devices, etc. are limited within the LANs (local area networks). Other communication, e.g., control and protection data exchanges between the intelligent electronic devices and the supervisory control and data acquisition (SCADA) system or centralized protection and control (CPC) system, often travels through WANs (wide area networks).
To fulfill stringent reliability requirements of communication while maintaining interoperability of intelligent electronic devices in substation automation systems (SASs), International Electrotechnical Commission (IEC) standard 62439-3 defines two redundancy protocols: Parallel Redundancy Protocol (PRP) (IEC 62439-3 Clause 4) and High-availability Seamless Redundancy (HSR) (IEC 62439-3 Clause 5). Both redundancy protocols can overcome failure of a link or a switch in a network with zero switchover time, while enabling clock synchronization according to IEEE 1588 (v2).
Both redundancy protocols employ different approaches and infrastructures. PRP duplicates the data frames to be transmitted, adds a redundancy control trailer (RCT) with a unique sequence number to the end each of a standard data packet such as an IP (Internet protocol) data packet for each PRP frame, and sends both PRP frames through two independent LANs having a similar-topology (IST-LANs). The receiver identifies the frames by the RCT and source MAC (media access control) address, accepts and processes the first-arrived PRP frame, and discards the second PRP frame if it ever arrives. Since the RCT is added at the end of standard data packet as part of the PRP frame, the RCT can be ignored by non-PRP compatible equipment. This approach ensures that PRP works with both PRP compatible and non-compatible equipment as long as the transmitter and receiver ends are PRP compatible.
Similarly, HSR duplicates a data frame and sends both data frames in opposite directions through a ring-topology local area network (RT-LAN). On the ring, each device incorporates a switch element that decides whether to forward or discard the frames from one port to the other. However, instead of adding redundancy information at the tail of the frame as is done in PRP, HSR inserts a header between the MAC header and payload of the data frame. Consequently, HSR-tagged data frames can be processed only by HSR-compatible network equipment, and dropped as bad frames by HSR non-compatible equipment.
HSR and PRP are designed for and benefit only communication within LANs since the source MAC address of the respective frames, used with a sequence number (assigned by HSR/PRP) as a unique doublet to make forward/discard decisions, will lose its uniqueness and be replaced by the MAC address of a different entity (e.g. router or layer-3 switch) when transferred from a LAN to a WAN (wide area network). Consequently, even though redundancy protocols can be enabled within a LAN at both ends in accordance with IEC 62439-3, communication through WANs could experience a long recovery time if LANs are connected to WANs with redundant routers, or even suffer single point failure if each LAN is connected to a WAN with a single router.
Therefore, there is a need to adapt LAN redundancy protocols, especially PRP, to support WAN communication with existing network structure.